Method for making a detachable semiconductor substrate and for obtaining a semiconductor element

ABSTRACT

The invention concerns a method for forming a semiconductor substrate that can be dismantled, comprising the following steps: 
         introduction of gaseous species in the substrate ( 1 ) according to conditions enabling the constitution of an embrittled layer by the presence in said layer of micro-cavities and/or micro-bubbles, a thin layer of semiconductor material thus being delimited between the embrittled layer and one face ( 2 ) of the substrate,    thermal treatment of the substrate to increase the brittleness level of the embrittled layer, said thermal treatment being continued until the appearance of local deformations on said face ( 2 ) of the substrate ( 1 ) in the form of blisters but without generating exfoliations of the thin layer during this step and during the continuation of the method, epitaxy of semiconductor material ( 6 ) on said face of the substrate to provide at least one epitaxial layer on said thin film.

TECHNICAL FIELD

The present invention concerns a method for forming a semiconductorsubstrate that can be dismantled. It further concerns a method forobtaining an element in semiconductor material.

STATE OF THE PRIOR ART

Document FR-A-2 681 472 (corresponding to U.S. Pat. No. 5,374,564)describes a method for producing thin films of semiconductor material.It teaches that the introduction of a rare gas or hydrogen into asubstrate is capable of inducing, under certain conditions, theformation of micro-cavities or micro-bubbles at a depth close to theaverage penetration depth of the implanted ions. By bringing intointimate contact this substrate, through its implanted face, with astiffener and carrying out a suitable thermal treatment, an interactionbetween the micro-cavities or micro-bubbles present in the implantedzone may take place. This interaction may then lead to a fracture at thelevel of the implanted zone of the semiconductor substrate. Two partsare then obtained: on the one hand a semiconductor film adhering to thestiffener and, on the other hand, the remainder of the initiallyimplanted substrate. The remainder of the substrate may be recycled andreused. This method makes it possible, in particular, to transfer a thinfilm of material onto a support substrate that may be of a differentnature. In particular, it enables the production of SOI substrates(“Silicon On Insulator”).

This process may also be applied to the production of a thin film ofsolid material other than a semiconductor material, such as for examplea conductor or dielectric material, crystalline or not, as disclosed indocument FR-A-2 748 850 (corresponding to U.S. Pat. No. 6,190,998).

Furthermore, document FR-A-2 748 851 (corresponding to U.S. Pat. No.6,020,252) discloses a method that makes it possible to includetechnological steps requiring the use of high temperatures, for example900° C., between the initial ion implantation step and the finalseparation step. These intermediate steps comprising thermal treatmentsmay be carried out without degrading the surface condition of the flatface of the wafer and without leading to fracture at the level of theimplanted zone, which would induce the separation of the upper thinfilm. They may, for example, be part of operations for formingelectronic components. In this case, the step of ion implantation, forexample of hydrogen, must be carried out within a range of suitabledoses. The final separation step may then be achieved either by thermaltreatment, or by application of a mechanical stress to the structure, oreven by combination of these two treatments.

Document FR-A-2 809 867 discloses a method comprising a step ofintroducing gaseous species along a buried layer in the substrate, whichwill lead to the formation of micro-cavities in said layer. The buriedimplanted zone thus embrittled delimits a thin film with one face of thesubstrate. The method further comprises a step of eliminating all orpart of the gaseous species from the embrittled zone. This evacuation ofthe gaseous phase allows subsequent treatments to be carried out at hightemperatures and this without inducing a deformation of the surface (forexample, in the form of blisters) or a total or partial separation ofthe upper layer. A step of over-embrittlement of the zone ofmicro-cavities may optionally be realised by carrying out appropriatethermal treatments, by the application of thermal stresses or even bycombination of these two types of treatments. This over-embrittlement ofthe buried zone may for example facilitate the final separation betweenthe upper layer of the substrate and the substrate. The step ofimplanting gaseous species is carried out in a range of suitable dosesthat makes it possible to achieve, firstly, a sufficient embrittlementof the buried zone and, secondly, the subsequent elimination of thegaseous species.

In document FR-A-2 758 907 (corresponding to U.S. Pat. No. 6,316,333),the method described makes it possible to transfer a layer of material,for example of silicon, on which it has been possible to formcomponents. The substrate comprising the processed layer is prepared insuch a way that certain zones on the surface are masked. The speciesintroduced into this substrate by an ion implantation step is thenlocalised in the non-masked zones. On the other hand, the masked zonesto not receive ions. At the level of the implanted zones, one thusobtains a buried layer of specific defects linked to the introduction ofgaseous species, such as micro-bubbles, micro-cavities or evenmicro-fissures. The masked zones may be chosen in such a way as toprotect the active zones of the components capable of undergoing adegradation during the crossing of the implanted ions. Typically, thedimensions of the masked zones are described as around 1 μm. Thesubsequent making integral of the substrate comprising the processedlayer and comprising locally implanted regions, followed by a separationtreatment, may in particular comprise an annealing at medium temperature(around 400° C.), then enables the active layer of components to betransferred onto a support substrate. Indeed, the development ofcavities and micro-fissures at the level of the implanted zones issuited to give rise to the separation over the full wafer of the upperlayer.

The results published by C. H. Yun et al. (“Transfer of patternedion-cut silicon layer”, Applied Physics Letters, vol. 73, no. 19,November 1998 and “Ion-cut silicon layer transfer with patternedimplantation of hydrogen”, Electrochemical Society Proceedings, vol.99-3, p 125) have shown that the zones masked during the implantationstep may result in squares or lines, the maximum dimensions of which arerespectively around 15 μm×15 μm and 15 μm×100 μm. The implanted zonesseparating these masked patterns must be at least 5 μm. The transfer ofthe upper layer onto a support may then take place, from a thermaltreatment or by application of mechanical forces external to thestructure. However, the authors note that the transferred upper layercomprises relief inhomogeneities at the level of the non implantedzones. The larger the dimensions (between 10 and 15 μm), the larger aresaid homogeneities. The authors explain that these irregular reliefsstem from the deviation of the fissure in the cleavage planes (111) ofthe substrate.

Another method also allows the formation of a substrate that may bedismantled, capable of supporting a high temperature treatment (around1100° C.) before the final dismantling of the processed upper layer.This method is based on a local implantation, made possible by the useof a mask at the moment of introducing the ions (hydrogen and/or otherspecies such as, for example, noble gases). The size of the implantedregions is devised in such a way that the subsequent thermal treatmentsfor embrittleing the buried zone and/or required for all or part of thesteps of forming components does not induce the degradation of thesurface. This constraint is linked to the subsequent steps ofelaborating components, for example microelectronic components, whichnecessitates a perfect surface condition.

Obtaining substrates that may be dismantled, in other words comprisingan upper layer delimited by the surface of the substrate and by a buriedembrittled zone, said substrates being compatible with theimplementation of technological steps intended to produce components andthat may require high temperature treatments, is of increasing interest.Photovoltaic applications have an additional requirement, which is theuse of an inexpensive method.

Document FR-A-2 748 850, cited above, proposes a method for embrittleinga substrate along a buried layer, by introduction at low dose of agaseous species (for example hydrogen). The implanted dose must bechosen in such a way that a thermal annealing does not induce surfacedeformation or exfoliation. Depending on the mechanical forces appliedto induce the separation, the stage of embrittlement attained at thelevel of the implanted zone may then turn out to be insufficient. It maythen be interesting to increase the brittleness level of the implantedzone.

According to the method described in document FR-A-2 809 867, citedabove, the introduction of a controlled dose makes it possible to bothembrittle the buried zone then to evacuate the gas, in order to limit apressure effect during a rise in temperature. One therefore has nodeformation or exfoliation of the surface during technological steps athigh temperatures. This technique necessitates a strict control of thedose and the homogeneity of the dose of implanted species. It may proveto be interesting to relax the technological constraints relative to anarrow window of implantation parameters.

Document FR-A-2 758 907, cited above, proposes a local introduction ofgaseous species after forming components in the upper layer of thesubstrate. The introduction of these species leads to the formation of adiscontinuous buried layer of micro-cavities capable of generating thefracture after making the processed substrate integral on a supportsubstrate. The substrate is therefore embrittled after carrying out thedifferent technological steps of producing the components. Theaccessible size of the zones to be masked (which correspond to theactive zones of the components) may prove limitative depending on thetargeted applications. For example, for components of dimensions ofseveral tens of μm to several hundreds of μm, this technique isdifficult to implement. Moreover, depending on the technology used forforming the components, the thickness of the active layer, in otherwords comprising the components, may reach several μm (for example,around 50 μm for photovoltaic cells in silicon). The introduction ofgaseous species at a considerable depth while at the same timeefficiently protecting the zones by masking may then prove to beawkward, especially due to the equipment that is necessary (specificimplanters, accelerator) and costly, for example for photovoltaicapplications.

A substrate that may be dismantled may also be formed according to themethod described above and using a mask at the moment of introducing theions. This method employs an intermediate masking step prior to theimplantation that will make it possible, by a lateral confinement of theburied micro-fissures, to significantly limit the appearance ofdeformations in the form of blisters on the surface of the substrate.The surface condition of the substrate is then perfectly compatible withdifferent steps of forming components, for example microelectroniccomponents. On the other hand, this method may have the disadvantage ofbeing costly, particularly in application fields such as thephotovoltaic field.

DESCRIPTION OF THE INVENTION

The present invention has been designed with the aim of reducing costs,but keeping as objective obtaining a processed upper layer that may beseparated or detached from its substrate without breaking ordeteriorating the structure.

The subject of the invention is therefore a method for forming asemiconductor substrate that can be dismantled, comprising the followingsteps:

-   -   introduction of gaseous species in the substrate according to        conditions enabling the constitution of an embrittled layer        through the presence in this layer of micro-cavities and/or        micro-bubbles, a thin film of semiconductor material thus being        delimited between the embrittled layer and one face of the        substrate,    -   thermal treatment of the substrate to increase the brittleness        level of the embrittled layer, said thermal treatment being        continued until the appearance of local deformations of said        face of the substrate in the form of blisters but without        generating exfoliations of the thin film during this step and        during the continuation of the method,    -   epitaxy of semiconductor material on said face of the substrate        to provide at least one epitaxial layer on said thin film.

The introduction of gaseous species may be carried out by ionimplantation or plasma immersion implantation.

Before the step of thermal treatment of the substrate, provision may bemade for a step of forming a thickener, the thickness of which issufficiently large so as not to generate exfoliations in the thin filmand sufficiently small so as to avoid the separation of the substrate atthe level of the embrittled layer during the step of thermal treatmentof the embrittled layer. The thickener may then be totally or partiallyeliminated before the epitaxy step.

Provision may be made for an additional step of subjecting the epitaxiallayer to at least one component forming step. This may be a step offorming photovoltaic components.

Provision may also be made for an additional step of forming aprotective layer on the epitaxial layer, said protective layer beingintended to protect the epitaxial layer from a chemical attack intendedfor the separation of the substrate at the level of the embrittledlayer.

A further aim of the invention is a method for obtaining an element ofsemiconductor material, characterised in that it comprises the followingsteps:

-   -   providing a semiconductor substrate that may be dismantled        obtained by the method of forming a semiconductor substrate that        may be dismantled as described above,    -   dismantling the semiconductor substrate that may be dismantled        by detaching this substrate at the level of the embrittled        layer, the detachment being either total to provide an element        of semiconductor material forming a membrane and consisting of        the thin film of semiconductor material and the epitaxial layer,        or partial to provide one or several elements of semiconductor        material forming one or several components and consisting of a        part of the thin film of semiconductor material and the        epitaxial layer.

Provision may also be made for a supplementary step of attaching theepitaxial layer onto a support before the dismantling step.

The detachment may result from the application of a tensile stressand/or a shear stress. It may result from the implementation of a stepof introducing additional gaseous species into the embrittled layer,then a step of mechanical stressing and/or thermal treatment of theembrittled layer. It may also result from the application of an openingstress at the level of the embrittled layer. It may also result from achemical attack of the embrittled layer. It may also result from acombination of these methods.

The step of providing a semiconductor substrate that may be dismantledmay comprise a substrate that has already been dismantled and that isobtained by the above method of forming a semiconductor substrate thatmay be dismantled and without prior surface conditioning.

BRIEF DESCRIPTION OF DRAWINGS

The invention will be more fully understood and other advantages andparticularities will become clear on reading the following description,given by way of example and in nowise limitative and by referring to theappended drawings, among which:

FIG. 1 is a cross-sectional view of a semiconductor substrate subjectedto an ion implantation intended to form a buried embrittled layer,according to the invention;

FIG. 2 is a cross-sectional view of the semiconductor substrate of FIG.1 having undergone a thermal treatment making it possible to increasethe brittleness level of the buried embrittled layer, according to theinvention;

FIG. 3 is a cross-sectional view of the semiconductor substrate of FIG.2 after the epitaxy of a layer of semiconductor material on all theimplanted face of the substrate, according to the invention;

FIG. 4 is a cross-sectional view of the semiconductor substrate of FIG.2 after the epitaxy of a layer of semiconductor material on a part ofthe implanted face of the substrate, according to the invention;

FIG. 5 is a cross-sectional view of the semiconductor substrate of FIG.3 after forming components in the epitaxial layer, according to theinvention;

FIG. 6 is a cross-sectional view of the semiconductor substrate of FIG.5 after attachment of the epitaxial layer on a support, according to theinvention;

FIG. 7 is a cross-sectional view of the semiconductor substrate fixed toits support, as shown in FIG. 6, during the step of separation,according to the invention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

The method according to the invention applies in particular to the fieldof photovoltaic applications. It enables a substrate to be formed (inparticular in silicon) comprising an embrittled buried layer, which mayundergo the different steps of epitaxial growth and elaboration ofcells, specific to the targeted photovoltaic application, then allow thefinal separation between the processed upper layer and the remainder ofthe substrate.

This method is based on the implantation of gaseous species such as ionsof hydrogen and/or noble gases, capable of creating, around the depth ofmaximum concentration, a zone embrittled by micro-cavities, “platelets”and/or micro-bubbles. A thermal treatment making it possible to increasethe brittleness level of the embrittled layer is carried out. Saidthermal treatment will henceforth be called embrittlement thermaltreatment. Under certain conditions, said embrittlement thermaltreatment imposed on the implanted substrate has the consequence offorming cavities and/or micro-fissures in the buried embrittled zoneleading to the appearance of blisters or bubbles on the surface of thesubstrate.

According to the method of the invention, the implantation conditions(the principal parameters of which are the energy, the dose and thetemperature) as well as the parameters of the embrittlement thermaltreatment and different, optional, subsequent steps of preparing thesubstrate (thermal treatments, deposition of layers, etc.) must bechosen as a function of the nature of the substrate (nature of thematerial, crystalline orientation, etc.), in such a way that noexfoliation of the upper layer appears. Exfoliation is taken to mean apartial detachment of the thin film at the level of the embrittled zone.In the same way, during the technological steps specific to the targetedapplication, for example during a step of epitaxial growth of the layerof silicon necessary for forming solar cells, no exfoliation will begenerated.

After all or part of the forming of the photovoltaic components, and incompliance with certain implementations described hereafter, theprocessed upper layer (around 50 μm) may be dissociated from the initialsubstrate by embrittlement at the level of the embrittled zone by themicro-cavities and/or micro-fissures.

The method begins by the creation of a buried embrittled layer that willallow, after technological steps have been carried out, the separationbetween an upper layer and the remainder of the substrate. It is basedon the introduction in the substrate of gaseous species such as hydrogenand/or other noble gases (helium, etc.) by a technique allowing acontrolled localisation of the species in depth (for example, ionimplantation, plasma immersion implantation, etc.). Such an implantationmakes it possible to form a buried embrittled zone typically composed ofmicro-cavities, “platelets” and/or micro-bubbles. This buried, disruptedzone delimits, with the surface of the substrate, an upper layer.

With the aim of increasing the brittleness level of the implanted zone,optional steps of preparing the substrate comprising for example, athermal treatment and/or a deposition of thickener may be carried out onthe substrate before the embrittlement thermal treatment.

Indeed, under thermal activation, the micro-cavities and/or “platelets”,generated during the implantation, for example of hydrogen ions, aregoing to follow a growth law. Thus, if the implanted dose is sufficient,the cumulative effect of the increase in the size of the cavities and/orthe micro-fissures and the gas pressure in them is the appearance oflocal deformations of the upper layer in the form of blisters.

The characteristics (morphology, size, density) of said micro-fissuresand/or cavities reflect the state of embrittlement of the implantedzone.

According to a variant of the embrittlement treatment, one may form adeposit of oxide on the surface of the implanted substrate before anythermal treatment. This will play a mechanical role of thickener, whichallows the micro-fissures to develop laterally along larger dimensions,under the effect of a suitable thermal annealing. Characteristics ofmicro-fissures favourable to a higher embrittlement may be obtained bythe addition of said deposited layer.

For a given substrate, the parameters of implantation, embrittlementthermal treatment and the parameters for conditioning the substrate(thermal treatments, deposition of layers acting as thickeners, etc.),determine the size and the density of the blisters on the surface of thesubstrate, in other words also the size and density of the underlyingmicro-fissures and/or cavities, contained within the implanted zone.

The level of embrittlement of the substrate can also be configured as afunction of the implantation conditions and the characteristics of thesubsequent treatments.

Thus, for a given substrate, it is necessary to choose the implantationconditions (energy, dose, temperature) in such a way that:

-   -   the carrying out of a suitable embrittlement treatment        effectively leads to obtaining micro-fissures of large        dimension, capable of generating blisters on the surface of the        substrate. However, this treatment must not in any case generate        exfoliations of the upper layer.    -   any subsequent treatment, linked to the technological steps        specific to the targeted application, for example epitaxy in the        range 550° C.-1100° C., does not induce the local exfoliation of        the layer.

The method according to the invention combines implantation conditionsand embrittlement treatments that make it possible to avoid the localexfoliation of the upper layer before and after the differenttechnological steps relating to the targeted application.

All of the implantation conditions and treatments enabling theembrittlement to be increased must make it possible to obtain asubstrate comprising localised micro-fissures in the implanted zone,giving rise to the appearance of blisters on its surface. Saidmicro-fissures are randomly distributed in the buried embrittlementplane and their size follows a Gaussian distribution law. This state ofembrittlement is such that any additional treatment imposed on thesubstrate, including in particular high temperature treatments, does notinduce local exfoliation of the upper layer delimited by the implantedzone and the surface of the substrate, for example at the level of oneor several micro-fissures.

It is important to note that the morphology, the size and the density ofthe micro-fissures and cavities present in the embrittled layer may bevariable depending on the subsequent treatments imposed on theembrittled substrate. The high thermal budgets for which thereconstruction of the material constituting the substrate issignificant, have a notable influence on the morphological change of themicro-fissures formed after the implantation, followed by anembrittlement treatment requiring a low thermal budget. After highthermal budgets, said micro-fissures and/or cavities evolve towardsstable polyhedral shapes. One notes in particular a polyhedric typereconstruction of the edges of fissures. Their sizes and densities alsostrongly depend on the thermal budget imposed on the embrittledsubstrate. One may note that the step of embrittlement may also comprisea high temperature thermal treatment, in such a way as to transform themicro-fissures and/or cavities present in the implanted zone into stableobjects (evacuation of all or part of the gaseous species and at leastpartial reconstruction of the edges of fissures). This has the aim ofavoiding any phenomenon of exfoliation during the technological steps ofepitaxy and/or forming of components.

At this stage, and in order to allow the elaboration of components, forexample photovoltaic components, the epitaxial growth of a thick layerof silicon (up to around 50 μm), may be carried out on the blisteredsubstrate. The “mild” relief of the surface blisters (typically of adiameter ranging from several μm to several tens of μm and in which themaximum deformation may reach several hundreds of nanometers) allowsthick layers of monocrystalline silicon of good quality for the targetedapplication to be obtained by epitaxy.

Different steps of elaborating photovoltaic type components may then becarried out.

The continuation of the method corresponds to the final separationbetween the upper layer and the initial substrate. This separation willtake place at the level of the embrittled buried zone.

This step may be carried out by application of a tensile stress, shearstress or mixed mode involving both tensile and shear stress, to theembrittled zone.

According to a first embodiment, the upper processed layer may beseparated from its source-substrate, thus generating a self-supportingmembrane. Said membrane may be separated over the whole surface area ofthe substrate or instead locally at the level of a component or anassembly of components, by application of an opening stress at the levelof the embrittled zone.

According to a second embodiment, a low cost support substrate could belinked by means of an adhesive layer (polymer, resin, ceramic, metallicor other, etc.) to the processed silicon layer. The separation, forexample by application of tensile and/or shear stresses and/or accordingto a mixed mode to the bonded structure may then be carried out at thelevel of the buried embrittled zone.

One then obtains:

-   -   the initial substrate on the one hand, peeled from a thin film        (said film being the zone delimited by the implanted zone and        the surface of the initial substrate),    -   and the support on the other hand, comprising the processed        membrane of semiconductor material (for example of silicon).

According to a variant of the method and in order to facilitate thefinal dissociation of the membrane of the initial substrate, a chemicalattack may be carried out. A SECCO® type solution has the property ofattacking the silicon and more preferentially the zones of stressedsilicon and/or having undergone plastic deformations and/or disrupted bythe presence of inclusions, structurings or other morphologicaltransformations. Thus, the embrittled zone comprising micro-fissuresand/or cavities, the lateral size of which may vary from around severaltens of nanometers to several tens of μm, will be a privileged zone ofattack of the SECCO® solution. In this way it is possible to initiate adissociation of the processed upper layer of the initial substrate, by aprogressive localised consumption by the chemical solution of the buriedembrittled zone. According to this variant of the separation step, itwill be necessary to carry out the deposition of a protective layer,particularly on the upper processed face of the substrate, in such a wayas to avoid the degradation of the cells and/or components formed.

Other solutions allowing the etching of the semiconductor material atthe level of the embrittled zone may also be used providing that theprocessed parts of the substrate are protected in an efficient manner.One may, in particular, cite TMAH and KOH if the semiconductor materialis of silicon.

According to a variant of the method, this step consisting in alocalised attack at the level of the embrittled zone, may be carried outbefore the forming of components (for example, photovoltaic cells).Thus, it will not be necessary to take precautions to protect theprocessed face of the embrittled substrate.

This technique of preferential chemical attack may be used alone tototally separate the processed membrane from its initial substrate, thiswith the aim of obtaining a self-supporting layer, or instead with theaim of transferring said membrane onto an inexpensive mechanicalsupport.

According to a variant, this technique of chemical attack may be usedjointly with a mechanical separation technique by application ofexternal forces such as, for example, tensile forces. In this case, thechemical attack will have the advantage of initiating and localising thefissure at the level of the buried embrittled zone and thus facilitatingthe final dissociation.

According to a variant, one could subject the substrate to treatments bysound waves (ultrasounds, nanovibrations, etc.).

According to a variant of the method, one may carry out a treatmentintended to diffuse the gaseous species, for example of hydrogen,through the front or rear face of the processed substrate. Thetechniques used may be, for example, plasma hydrogenation, or othermethods for introducing gaseous species by diffusion. This step mayenable the brittleness level of the buried zone of micro-fissures and/ormicro-cavities to be increased. The species introduced into the materialare preferentially trapped at the level of the micro-fissures zone.

The final separation may then be realised by a suitable thermaltreatment and/or by application of a suitable mechanical stress and/orby a treatment comprising both a thermal treatment and a mechanicaltreatment.

The initial substrate peeled from an upper layer may then be recycled inorder to form another substrate to be embrittled.

It should be noted that, in an advantageous manner, no surfaceconditioning treatment (for example, polishing) will be necessary onsaid recycled substrate. The introduction of gaseous species may becarried out directly in the substrate, the surface of which has amicro-roughness linked to the dissociation step.

FIG. 1 is a cross-sectional view of a semiconductor substrate 1, in thisexample it is a substrate in silicon, which is going to be subjected tothe method of the invention, according to a first embodiment. Theprincipal face 2 of the substrate of silicon 1 is covered with a layerof silicon oxide 3.

An ion implantation of hydrogen, symbolically represented by arrows inFIG. 1, is carried out in the substrate 1 through the oxide layer 3. Theimplantation beam has an energy of 210 keV and a dose of 6.10¹⁶H⁺/cm².It gives rise to the formation of a buried embrittled layer 4 comprisingmicro-bubbles and micro-cavities.

An annealing at 550° C. for 30 minutes is carried out on said substratein order to increase the brittleness level of the implanted zone.Indeed, under thermal activation, the cavities induced by theimplantation follow a law of growth to form micro-fissures and/orcavities of greater size that give rise to the appearance of blisters 5on the surface of the substrate as shown in FIG. 2.

The oxidised surface of the substrate is then deoxidised, then atechnological step of liquid phase or vapour phase silicon epitaxy iscarried out. FIG. 3 shows the presence of an epitaxial layer 6 on theprincipal face 2 of the substrate 1. The thickness of the epitaxiallayer 6 may be 50 μm. The liquid phase epitaxy may be carried out at atemperature of around 950° C. for 2 hours. The vapour phase epitaxy maybe carried out around 1100° C. for 1 hour.

According to a second embodiment of the invention, the ion implantationof hydrogen is carried out for an energy of 76 keV and a dose of6.10¹⁶H⁺/cm² through a 400 nm layer of silicon oxide covering a siliconsubstrate. A PECVD deposition is then carried out to obtain anadditional layer of silicon oxide 3 μm thick. This substrate may thenundergo an embrittlement treatment such as an increasing temperatureannealing going from 400° C. up to 1100° C., the rise in temperaturebeing, for example, 3° C./min and the time of treatment at 1100° C.being one hour. This treatment has the effect of making the cavitiesand/or micro-fissures grow at the level of the implanted zone then ofstabilising these cavities and/or micro-fissures at high temperature sothat no modification takes place of the state of the surface conditionof the substrate during the removal of the thick layer of oxide orduring subsequent technological steps.

The oxidised surface of the substrate is then deoxidised, prior to atechnological step of liquid or vapour phase epitaxy of silicon, of athickness between 20 μm and 50 μm.

According to a third embodiment of the invention, the implantation ofhydrogen is carried out by plasma immersion for an energy of 76 keV anda dose of 6.10¹⁶H⁺/cm² through a layer of silicon oxide of 400 nmcovering a silicon substrate. A PECVD deposition is then carried out toobtain an additional layer of silicon oxide 3 μm thick. This substratemay then undergo an embrittlement treatment such as an increasingtemperature annealing going from 400° C. up to 1100° C., the rise intemperature being, for example, 3° C./min.

The oxidised surface of the substrate is then partially deoxidised. Theoxide is for example removed from the central part of the substrate butremains on a crown on the edge of the substrate, of a width that mayvary from several hundreds of μm to several mm. A step of liquid phaseepitaxy to obtain an epitaxial layer of thickness between 20 μm and 50μm is then carried out.

FIG. 4 illustrates this third embodiment of the invention. It shows asubstrate in silicon 11 comprising a buried embrittled layer 14, a layerof oxide 13 remaining as a crown on the substrate 11 and an epitaxiallayer 16 deposited on the part of the face 2 of the substrate notcovered with oxide. It can be clearly seen that on the crown 13 theresumption of epitaxy has not taken place.

According to a fourth embodiment of the invention, the ion implantationof hydrogen is carried out for an energy of 210 keV and a dose of7.10¹⁶H⁺/cm² through a layer of silicon oxide of 200 nm covering asilicon substrate. A deposition by PECVD is then carried out to obtainan additional layer of silicon oxide of 10 μm thickness.

This substrate may then undergo an embrittlement annealing at 450° C.for 14 hours, followed by an increase at 3° C./min up to 1100° C.

The oxidised surface of the substrate is then partially deoxidised. Theoxide is for example removed from the central part of the substrate butremains on a crown at the edge of the substrate, of a width that mayrange from several hundreds of μm to several mm. A step of vapour phaseepitaxy of silicon to obtain an epitaxial layer of thickness between 20and 50 μm is then carried out.

After this step, the substrate comprises a central part on which theresumption of epitaxy of monocrystalline silicon has been possible dueto the fact of the de-oxidation prior to the epitaxy. The substrate thuscomprises, on its peripheral part, the crown of silicon oxide on whichthe resumption of epitaxy has taken place in polycrystalline form.

According to a fifth embodiment of the invention, the implantation ofhydrogen by plasma immersion is carried out for an energy of 76 keV anda dose of 6.10¹⁶H⁺/cm² through a 400 nm layer of silicon oxide coveringa double faced polished silicon substrate. The implantation is carriedout on the two principal faces of the substrate. A PECVD deposition ofoxide of 3 μm is carried out on said double faced implanted substrate.This substrate then undergoes an embrittlement treatment such as anincreasing temperature annealing going from 400° C. up to 1100° C., therise in temperature being, for example, 3° C./min.

The oxidised surfaces of the substrate are then partially deoxidised. Inparticular, the oxide is removed from a central part of the two faces ofthe substrate but remains on a crown on the edge of a width that mayvary from several hundreds of μm to several mm. A technological step ofliquid phase epitaxy, of a thickness between 20 μm and 50 μm is thencarried out, on the two faces of the substrate. After this step, thesubstrate comprises a central part on which the resumption of epitaxyhas been possible due to the fact of the de-oxidation prior to theepitaxy. The substrate comprises, on the peripheral part of its faces,the deposited crown of PECVD oxide on which the resumption of epitaxyhas not been possible.

According to a sixth embodiment of the invention, the ionic implantationof hydrogen is carried out for an energy of 52 keV and a dose of5.5.10¹⁶ H⁺/cm² through a 200 nm layer of silicon oxide covering asilicon substrate. A PECVD deposition is then carried out to obtain anadditional layer of oxide 5 μm thick. The substrate may then undergo anembrittlement annealing at 500° C. for 4 hours. The oxidised surface ofthe substrate is then totally deoxidised. A liquid phase epitaxy step at600° C. may then be carried out with the aim of obtaining an epitaxiallayer of between 20 μm and 50 μm

Whatever the embodiment of the invention, technological steps of formingcomponents, for example photovoltaic components, may be implemented.FIG. 5 shows components 7 formed in the epitaxial layer 6 of thesubstrate represented in FIG. 3. If the epitaxy has been carried out onthe two principal faces of the substrate, components may then be formedin the two epitaxial layers.

The substrate thus processed is then made integral by means of a ceramicadhesive 8, for example on a mechanical support 9 such as a ceramic,glass or mullite substrate as shown in FIG. 6. The insertion in theregion of the embrittled layer 4 of a suitable tool comprising forexample a bevelled blade will then enable a cut to be made at the levelof the buried implanted zone, thus separating the processed layer 6 fromits initial substrate 1, and leaving it integral with the substratesupport 9. The initial substrate 1 may then be recycled into a newsubstrate to be embrittled.

The processed substrate of the second embodiment of the invention may bemade integral by means of a polymer adhesive on a mechanical supportsuch as a plastic substrate. The immersion of the bonded structure in asolution, for example of SECCO®, provokes the preferential attack of thepolycrystalline silicon, the crown of oxide and the buried embrittledzone, which may initiate the separation between the processed layer fromits initial substrate. The separation may take place completelyaccording to this chemical route or instead be assisted by theapplication of a mechanical stress or instead be relayed by a purelymechanical opening route using tensile and/or shear stresses asindicated by the arrows F in FIGS. 6 and 7. The initial substrate maythen be recycled into a new substrate to be embrittled.

The processed substrate of the third embodiment of the invention may bemade integral by means of a polymer adhesive on a mechanical supportsuch as a plastic substrate. The bonded structure may then be immersedin a bath of HF. This has the effect of attacking the peripheral layerof oxide present on the processed embrittled substrate. Thus, theapplication of the stress during the insertion of a blade is betterlocalised at the level of the embrittled zone. One then ends up with theseparation between the processed layer and its initial substrate. Theinitial substrate could then be recycled into a new substrate to beembrittled.

The processed substrate of the fourth embodiment of the invention may bemade integral by means of a polymer adhesive on a mechanical supportsuch as a plastic substrate. The immersion of the bonded structure in asolution, for example of SECCO®, provokes the preferential attack of thepolycrystalline silicon, the crown of oxide and the buried embrittledzone, which may initiate the separation between the processed layer fromits initial substrate. The separation may take place completelyaccording to this chemical route or instead be assisted by theapplication of a mechanical stress or instead be relayed by a purelymechanical opening route using tensile and/or shear stresses asindicated by the arrows F in FIGS. 6 and 7. The initial substrate maythen be recycled into a new substrate to be embrittled.

The processed substrate of the fifth embodiment of the invention may bemade integral by means of a polymer adhesive on a mechanical supportsuch as a plastic substrate. The bonded structure is then immersed in abath of HF. This has the effect of attacking the peripheral oxide layerpresent on each of the faces of the processed embrittled substrate. Theapplication of the stress during the insertion of a blade will be betterlocalised at the level of the embrittled zone. This stress will besuccessively or jointly applied in the region of the embrittled layer ofone or other face of the substrate. One then ends up with the separationbetween the processed layers and the initial substrate. The initialsubstrate may then be recycled into a new substrate to be embrittled.

1. Method for forming a semiconductor substrate that may be dismantled,comprising the following steps: introduction of gaseous species in thesubstrate (1) according to conditions allowing the constitution of anembrittled layer (4) by the presence in said layer of micro-cavitiesand/or micro-bubbles, a thin film of semiconductor material thus beingdelimited between the embrittled layer (4) and one face (2) of thesubstrate, thermal treatment of the substrate to increase thebrittleness level of the embrittled layer (4), said thermal treatmentbeing continued until the appearance of local deformations on said face(2) of the substrate (1) in the form of blisters but without generatingexfoliations of the thin film during this step and during thecontinuation of the method, epitaxy of semiconductor material (6) onsaid face of the substrate to provide at least one epitaxial layer onsaid thin film.
 2. Method according to claim 1, characterised in thatthe introduction of gaseous species is carried out by ion implantationor plasma immersion implantation.
 3. Method according to claim 2,characterised in that the introduction of gaseous species is carried outby plasma immersion implantation, the method is applied on two faces ofthe substrate.
 4. Method according to claim 1, characterised in that,before the step of thermal treatment of the substrate, it provides for astep of forming a thickener, the thickness of which is sufficientlylarge so as not to generate exfoliations in the thin film andsufficiently small to avoid the separation of the substrate at the levelof the embrittled layer during the step of thermal treatment of thesubstrate.
 5. Method according to claim 4, characterised in that thethickener is totally or partially eliminated before the epitaxy step. 6.Method according to claim 1, characterised in that it provides for anadditional step of subjecting the epitaxial layer to at least one stepof forming components (7).
 7. Method according to claim 6, characterisedin that said step of forming components (7) is a step of formingphotovoltaic components.
 8. Method according to any of the previousclaims, characterised in that it provides for an additional step offorming a protective layer on the epitaxial layer, said protective layerbeing intended to protect the epitaxial layer from a chemical attackintended for the separation of the substrate at the level of theembrittled layer.
 9. Method for obtaining an element of semiconductormaterial, characterised in that it comprises the following steps:providing a semiconductor substrate that may be dismantled obtained bythe method according to any of claims 1 to 8, dismantling of thesemiconductor substrate that may be dismantled by detachment of thissubstrate at the level of the embrittled layer, the separation beingeither total to provide an element of semiconductor material forming amembrane and consisting of the thin film of semiconductor material andthe epitaxial layer, or partial to provide one or several elements ofsemiconductor material forming one or several components and consistingof a part of the thin film of semiconductor material and the epitaxiallayer.
 10. Method according to claim 9, characterised in that itprovides for an additional step of fastening of the epitaxial layer ontoa support before the step of dismantling.
 11. Method according to claim9, characterised in that the detachment results from the application ofa tensile stress and/or a shear stress.
 12. Method according to claim 9,characterised in that the detachment results from the implementation ofa step of introducing additional gaseous species into the embrittledlayer, then a step of mechanical stress and/or thermal treatment of theembrittled layer.
 13. Method according to claim 9, characterised in thatthe detachment results from the application of an opening stress at thelevel of the embrittled layer.
 14. Method according to claim 9,characterised in that the detachment results from a chemical attack ofthe embrittled layer.
 15. Method according to claim 9, characterised inthat the detachment results from a treatment by sound waves of theembrittled layer.
 16. Method according to any of claims 9 to 15,characterised in that the step of providing a semiconductor substratethat may be dismantled comprises providing a substrate that has alreadybeen dismantled and that is obtained by the method according to any ofclaims 1 to 8 with prior surface conditioning.